Structures at the Source Physics Experiment Phase II Site and Their Effects on Shear Wave Generation

Abstract:

Shear wave generation from explosive sources is an important problem for the study of the source mechanism and discrimination of explosions from earthquakes. In this study, we use a well-controlled and instrumented series of chemical explosions in Nevada, the Source Physics Experiment (SPE), to understand the mechanism of shear wave generation. We focus on SPE Phase II located at an alluvium site, and the structure effects on shear wave generation there. Using data recorded by a dense network of seismic stations, we conducted first arrival time analysis, Rg wave analysis, and horizontal-to-vertical spectral ratio analysis to image the geological structure. Our results show clear layers of alluvium, volcanic tuff, and Paleozoic basement. Velocity variation is observed at different scales. To understand how much of shear energy is produced within the 3D subsurface, we performed full waveform modeling using the spectral element code SPECFEM3D. We further varied the velocity of each geological unit in the model to account for uncertainties and random heterogeneities. We quantify the shear energy in the modeled wavefield by measuring the amplitude of the transverse component and P/S ratio. This estimation of shear wave generation can be compared with observations to understand the important partitioning of seismic energy from explosions and validate our model of the subsurface.